Vibrational entropy of crystalline solids from covariance of atomic displacements

TL;DR

This paper introduces an information-theoretic approach to calculate vibrational entropy in crystalline solids using atomic displacement covariances from AIMD simulations, improving accuracy over traditional methods especially at low temperatures.

Contribution

The authors develop a novel entropy calculation method based on atomic displacement covariances, extending applicability beyond quasiharmonic approximations and to complex materials.

Findings

Accurate entropy estimates for sodium and aluminum above Debye temperatures.

Effective vibrational density of states derived from covariance eigenvalues at low temperatures.

Method successfully applied to high entropy alloy and Ti phase transition.

Abstract

The vibrational entropy of a solid at finite temperature is investigated from the perspective of information theory. Ab initio molecular dynamics (AIMD) simulations generate ensembles of atomic configurations at finite temperature from which we obtain the $N$-body distribution of atomic displacements, $\rho_N$. We calculate the information-theoretic entropy from the expectation value of $\ln{\rho_N}$. At a first level of approximation, treating individual atomic displacements independently, our method may be applied using Debye-Waller B-factors, allowing diffraction experiments to obtain an upper bound on the thermodynamic entropy. At the next level of approximation we correct the overestimation through inclusion of displacement covariances. We apply this approach to elemental body-centered cubic sodium and face-centered cubic aluminum, showing good agreement with experimental values above the Debye temperatures of the metals. Below the Debye temperatures we extract an effective vibrational density of states from eigenvalues of the covariance matrix, and then evaluate the entropy quantum mechanically, again yielding good agreement with experiment down to low temperatures. Our method readily generalizes to complex solids, as we demonstrate for a high entropy alloy. Further, our method applies in cases where the quasiharmonic approximation fails, as we demonstrate by calculating the HCP/BCC transition in Ti.